New specimens of Bunaia woodwardi Clarke, 1919 (Euchelicerata): a new member of Offacolidae providing insight supporting the Arachnomorpha

Abstract

The rapid early diversification of arthropods has made understanding internal relationships within the group fiendish. Particularly unresolved is the origin of Euchelicerata, a clade consisting of the Prosomapoda (comprising the extant Xiphosura and Arachnida and the extinct Chasmataspidida, Eurypterida and synziphosurines) and the extinct Offacolidae. Here we describe new material of the Silurian 'synziphosurine' Bunaia woodwardi Clarke, 1919 that reveals previously unknown features of its ventral anatomy: a pair of elongated chelicerae in the prosoma, followed posteriorly by five pairs of biramous appendages, a first pre-abdomen somite bearing a pair of paddle-like uniramous appendages (exopods) and a ventral pretelsonic process. Phylogenetic analyses retrieve B. woodwardi as a member of Offacolidae closely related to Setapedites abundantis from the Early Ordovician Fezouata Biota. An anatomical comparison of the pretelsonic process of B. woodwardi, also present in Setapedites, with the posterior trunk morphologies of other Offacolidae, Habeliida and Vicissicaudata, suggests a possible homologous appendicular origin. This proposed apomorphic character supports a monophyletic Arachnomorpha, formed of Vicissicaudata, Habeliida and Euchelicerata. The establishment of this new homology could help to clarify the highly enigmatic phylogeny at the base of the euchelicerates as well as the sequence of character acquisition during their early evolution.

Keywords: Arachnomorpha; Artiopoda; Euchelicerata; Offacolidae; Vicissicaudata; synziphosurine.

Figure 1.

New specimen of B. woodwardi ROMIP53886. (a) Optical photograph under polarized filter and submerged in alcohol. (b) Interpretative line drawing. Highlighted in grey are the chelicerae, in green prosomal exopods, in blue endopods, in red opisthosomal exopods and in purple the pretelsonic process. (c) Multi-spectral imaging. (d) Multi-spectral imaging close-up from the upper boxed area in (c). Abbreviations: en2−5, endopods 2−5; ex2−6, exopods 2−6; mso, median sensory organs; ps, pleural spine; t1–t11, opisthosomal tergites 1−11; t, telson. Scale bars = 1 cm.

Figure 2.

New specimen of B. woodwardi ROMIP51447. (a) Normal light optical photograph. (b) Multi-spectral imaging. (c) Interpretative line drawing. Highlighted in grey are the chelicera, in green prosomal exopods, in blue endopods and in aquamarine unidentified exopods or endopods of appendages 4−6. Abbreviations: ap4−6, appendages 4−6; en2−3, endopods 2−3; ex2−3, exopods 2−3. Scale bars = 5 mm.

Figure 3.

New specimens of B. woodwardi ROMIP50689 and ROMIP50690. (a–c) Normal light optical photograph (a), interpretative line drawing (b) and multi-spectral imaging (c) of specimen ROMIP50689. (d–f) Normal light optical photograph (d), interpretative line drawing (e) and multi-spectral imaging (f) of specimen ROMIP50690. Highlighted in aquamarine in (b) are unidentified exopods or endopods of the appendages, and in yellow, in (e) are remains of the digestive system. Abbreviations: ap, appendages; ax1−6, axis 1−6 of the opisthosomal tergites; d, doublure; gs, genal spine; gu, gut; mr, cardiac (median) ridge; tp, apical tip. Scale bars = 1 cm (a–c), 5 mm (d–f).

Figure 4.

Lectotype and paralectotype of B. woodwardi. (a) Paralectotype NYSM 9910. (b) Paralectotype NYSM 9911. (c) Lectotype NYSM 9909. (d) Paralectotype NYSM 9912 (all photos credited to Russell Bicknell). Scale bars = 1 cm.

Figure 5.

Pretelsonic processes of B. woodwardi and S. abundantis (Offacolidae). (a,b) Multi-spectral imaging (a) and optical photography (b) close-ups of the pretelsonic process of B. woodwardi specimen ROMIP53886 from the boxed areas in figure 1c,a, respectively. (c) Interpretative line drawing of the pretelsonic process of B. woodwardi specimen ROMIP53886 based on (a,b). (d–f) Habitus view of S. abundantis specimen MGL102902 (d), and close-up (e) and interpretative line drawing (f) of its pretelsonic process. (g–i) Habitus view of S. abundantis specimen YPM IP 517932 (g), and close-up (h) and interpretative line drawing (i) of its pretelsonic process. Abbreviations: in, insertion; bf, bifurcation; cn, centrum. The different pretelsonic processes are highlighted in purple. Scale bars = 1 cm (a–c), 1 mm (d–i).

Figure 6.

Phylogenetic position of B. woodwardi among euchelicerates. (a) Maximum clade credibility tree of Bayesian analysis of euchelicerates. Numbers next to nodes are posterior probabilities when less than 100. (b) Implied weights K = 12 maximum parsimony tree of euchelicerates. Numbers next to nodes are relative Bremer support. Both trees are based on a matrix of 40 taxa and 114 characters modified by Lamsdell [12] and Lustri et al. [8].

Figure 7.

Appendicular derivatives of Emeraldella brutoni (Vicissicaudata) and the anal pouch of H. optata. (a–c) Habitus view of E. brutoni specimen UMNH.IP.6162 (a), and close-up (b) and interpretative line drawing (c) of its pretelsonic appendages. (d–f) Habitus view of H. optata specimen ROMIP64359 (d), and close-up (e) and interpretative line drawing (f) of its anal pouch. Abbreviations: cn, centrum. Appendicular derivatives are highlighted in purple. Photography credits: Javier Ortega-Hernández (a,b), (c) Royal Ontario Museum, Jean-Bernard Caron. Scale bars = 5 mm.

Figure 8.

Appendicular derivatives of Aglaspidida. (a–c) Habitus view of Glypharthrus tridentatus specimen NIGPAS165042a (a), and close-up (b) and interpretative line drawing (c) of its pretelsonic appendages. (d–f) Habitus view of A. spinifer specimen MPM11155 (d), and close-up (e) and interpretative line drawing (f) of its pretelsonic appendages (post-ventral plate). The different appendicular derivatives are highlighted in purple. Photography credits: Xuejian Zhu (a,b), Javier Ortega-Hernández (d,e). Scale bar = 1 mm (a–c), 2 mm (d–f).

Figure 9.

Schematic reconstructions of different posterior trunk morphologies of Habelia, Vicissicaudata and Offacolidae plotted onto a hypothetical phylogenetic relationship of Arachnomorpha. Phylogenetic tree based upon the present study results and refs. [4,8,12,36]. Schematic reconstructions drawn from refs. [8,19,23]. The different appendicular derivatives are highlighted in purple.